35 Marcus Blvd Hauppauge NY 11788
15161273 3100 Fax (5161 231-6004
TECHNICAL NOTE 7-5
January 1 992
RS-485 CABLING GUIDELINES FOR THE
COM20020 UNIVERSAL LOCAL AREA
NETWORK CONTROLLER (ULANC)
EXPERIMENTAL PROCEDURE FOR
VERIFICATION OF RS-485
By: Shaun Knoll, Systems Engineer and
Stephanie Colo, Project Engineer
TABLE OF CONTENTS
INTRODUCTION TO EIA RS-485 1
CABLING GUIDELINES FOR RS-485 INTERFACE WITH THE COM20020 1
TRANSMISSION LINE EFFECTS IN LOCAL AREA NETWORKS 2
Signal Attenuation 4
D.C. Loading 5
CALCULATING THE NUMBER OF NODES AND LENGTH OF CABLE .... 8
BIASING THE NETWORK 9
CAPACITIVE EFFECTS 11
CABLING TOPOLOGY AND CONNECTORS 11
EXPERIMENTAL VERIFICATION OF RS-485 CABLING GUIDELINES 13
5 Mbs DC COUPLED RS-485 ANALYSIS 19
INTRODUCTION TO EIA RS-485
EIA RS-485 is a specification for the support of a multi-drop differential serial digital
data network. RS-485 came about as microprocessors and the use of distributed
intelligence became popular in the design of industrial systems. The implementation
of such concepts created a demand for a standard method of communicating serially
in such environments. The actual RS-485 specification came about as a result of the
shortfalls of the RS-422 standard. RS-485 is virtually identical to RS-422 except in
two respects, increased receiver sensitivity and support of longer line lengths.
The basic RS-485 specification standardizes the electrical characteristics of each
transceiver and provides some basic guidelines for establishing a network. By
definition, a basic transceiver shall have a minimum input resistance of 1 2 Kohms and
handle a + 1-1 V common mode voltage regardless of whether power is applied or not.
When the transceiver is powered it must present the minimum input resistance and
present less than 50pf of capacitance at its input terminals. In addition, each driver
must be capable of providing a minimum level of 1.5V in the presence of 32
transceivers and two 1 20 ohm terminating resistors. The 1 20 ohm termination results
from the use of twisted pair cable, the preferred media in many industrial applications
because of its wide availability and low cost. Another critical requirement of RS-485
is that the receiver must be capable of detecting levels down to 200mV which is of
great advantage when long line lengths are needed. RS-485 does not specify a
modulation method or a maximum data rate. This gives the system designer great
flexibility in creating a low-cost high-performance network.
The combination of long line length, high node count (32 nodes), and support of low-
cost media has made the RS-485 specification the primary choice as a data
communication standard in industrial applications.
CABLING GUIDELINES FOR RS-485 INTERFACE
WITH THE COM20020
The following cabling guidelines provide a basis for establishing a low cost Local Area
Network (LAN) based on the ARCNET protocol for use with the COM20020 Universal
ARCNET Controller with a differential RS-485 driver. The guidelines presented are for
unshielded twisted pair cable modulated with the C0M20020's backplane encoding
scheme. All testing and experiments were performed using a 24AWG copper twisted
unshielded 2 pair cable with a characteristic impedance (Z ) of 120 Ohms. The
topology used in all experiments was a daisy-chained configuration with no stubs (i.e.
TRANSMISSION LINE EFFECTS IN LOCAL AREA NETWORKS
Transmission line effects often present an obstacle in obtaining high performance in
data communication networks. Among the problems that plague high data rate LAN's
are reflections, signal attenuation, and D.C. loading. Taking into account all three
parameters when designing a network can result in a faster and more reliable network.
A) REFLECTIONS IN TRANSMISSION LINE
A reflection in a transmission line is the result of an impedance discontinuity that a
travelling wave sees as it propagates down the line. To eliminate the presence of
reflections from the end of the cable you must terminate the line at its characteristic
impedance by placing a resistor across the line as shown in Figure 1 .
TERMINATION OF A NETWORK
Reflections are caused by a discontinuity in the line
DIRECTION OF PROPAGATION _
, , INCIDENT WAVE
7 = 120 ohm h
SOURCE (j~L\ REFLECTED WAVE .>R = 50OHM
Z = 120 OHM
DIRECTION OF PROPAGATION
1 Z = 120 ohm I 1 DISCONTINUITY
PROPERLY TERMINATED NETWORK
I 1 INCIDENT WAVE
7 = 120 ohm h
R = 120 OHM P» ( ri) DRIVER ]>R = 120OHM
— I 7 = 120 ohm I
It is important that the line be terminated at both ends since the direction of
propagation is bidirectional. In the case of unshielded twisted pair this termination is
120 ohms. Note that all reflection measurements were made with no stubs on the
network (i.e. no drops).
Theoretically, a properly terminated transmission line would produce no reflections at
all. However in a real network, small reflections are produced since the characteristic
impedance of the cable cannot be met exactly due to variance in the manufacturing
process of the cable. Another primary source of reflections is the impedance
mismatch between a data transceiver and the line, which can cause problems on a
data network by creating perturbations on the line during an otherwise idle state.
Reflections affect the network by triggering false transitions (bits) on the line
receiver's input translating into possible framing errors and CRC errors.
A measure of the relative strength of the reflection generated by discontinuities along
the line is called the Reflection Attenuation Factor (RAF). This is a measure of the
strength of the reflected wave to its incident wave. The RAF can be obtained by
comparing a reflected wave to its incident wave. The magnitude of the reflected
wave can be measured by sending a burst of sine waves down a transmission line and
observing at the sending end the magnitude of the wave after the burst has ended
(see Figure 2). The reflection measured at the sending end is the reflection generated
by a discontinuity at the receiving end of the line. The magnitude of the incident
wave can be measured at the receiving end of the cable, since this is the wave from
which the reflection is generated. It is important to compensate for line loss when
measuring the reflected wave because the reflected wave, measured at the sending
end of the cable, has lost some amplitude due to line loss.
MEASURING REFLECTIONS IN A NETWORK
1 Z = 120 Ohm
Z = 120 Ohm
Z = 120 Ohm"
R T = 120
Z = 120 Ohm
MEASURE REFLECTION HERE
MEASURE INCIDENT WAVE HERE
Measurements were made for twisted pair cable and are summarized in Table 1 . The
following relationship was used in calculating the Reflection attenuation.
Reflection attenuation = 20 log (V r8f /V inc )
where V ref = reflected voltage (compensated for loss)
V inc = incident voltage measured at receiving end of line
These above numbers can be interpreted as follows:
Assume a +5Vp_ p incident wave at 2.5Mhz, a reflected wave will be generated that
travels to the incident source at an amplitude of:
-24.52dB = 0.059
therefore the reflected wave is 0.059 * 5V = 0.297V.
In practice, the amplitude of the reflected wave might be smaller because
discontinuities are generated throughout the line and are not in phase with each other,
thus providing a canceling effect and decreasing the magnitude of the reflection.
There are several methods for minimizing the effects of reflections such as squelch
circuits and D.C. biasing. For the small reflection levels observed during
experimentation, the recommended choice is D.C. biasing for its simplicity and
minimum parts count (2 resistors). Biasing the network may cause some duty cycle
distortion but the ARCNET protocol is insensitive to duty cycle or jitter. The biasing
network will be discussed later in this guide.
B) SIGNAL ATTENUATION IN TRANSMISSION LINES
A second transmission line effect that has a bearing on the performance of LAN's is
signal attenuation. A transmission line can be modeled as a combination of the
distributed capacitance of the line, the distributed inductance, and resistance (see
TRANSMISSION LINE MODEL
FT = UNIT RESISTANCE OF LINE
L" = UNIT INDUCTANCE
C = UNIT CAPACITANCE
G' = UNIT ADMITTANCE
The capacitance of the line is formed by the parallel conductor pair. At the distances
used in LAN's (100's feet), the resistance of the cable is negligible and contributes
very little to line loss. The majority of line loss comes from the LC combination that
acts like a low pass filter and tends to attenuate the signal as frequency and distance
go up. For twisted pair cable, the attenuation rate is given in Table 2. These are
Table 2 - Signal Attenuation
C) D.C. LOADING IN RS-485 NETWORKS
The third parameter that affects network performance is D.C. loading. The D.C. load
presented to a line driver is a combination of three parameters:
1) loading effect of termination resistors
2) loading effect of biasing resistors
3) loading effect of RS-485 transceivers
EIA RS-485 specifies that a line driver must be capable of presenting a 1.5V signal
differentially at its outputs under the loading of 32 receivers and two 120 ohm
termination resistors. Each receiver or passive transceiver is to provide a minimum
input impedance of 12K ohms. The total parallel combination load impedance is 51
ohms, which includes the receiver load, termination resistors, and biasing resistors.
Since the worst-case specification is restrictive, it may not be practical to design to
this worst-case because, quite often the typical RS-485 transceiver can drive
substantially more than the worst-case load. Typically, many RS-485 drivers can
drive quite a bit more than the 51 ohms, sometimes as low as 20 ohms. If typical
characteristics are used, a network can be composed of many more nodes than the
32 specified by EIA RS-485. From laboratory experience, SMC recommends using
typical transceiver characteristics when determining the D.C. load. In very high
reliability applications (i.e. medical electronics, avionics), SMC recommends the use
of the worst case RS-485 parameters.
The D.C. biasing of the network is essential to providing reliable operation at high
data rates. The D.C. bias offsets the NULL voltage of the network when all RS-485
transceivers are in their TRI-STATE mode. A small offset is needed because small
reflections will occur that will cause spurious transitions on the RS-485 receivers
input. This would cause the C0M20020 to receive an undesired extra bit, thus
causing framing or CRC errors and corrupting the data. It is only the negative half of
the reflected wave that is of concern, so the effective reflection is .1 5V. The largest
reflections seen on the network are approximately 0.3Vp_ p differentially. This is more
than enough voltage to trigger a transition on the receiver's input since the receiver
has a hysteresis of 50mV and is thus quite sensitive (200mV differential sensitivity).
By installing the biasing network of Figure 4 at each node, the network will be offset
enough to keep the network in an inactive state. The + 0.35V differential offset
provided, will keep the line in an inactive state even in the presence of a 0.3Vp_ p
reflection (see Figure 5) which could cause a false bit to be recognized by the 20020
and cause data rejection by the receiving node. SMC recommends a pull-up and pull-
down resistor (see Table 3 for correct value) in the circuit diagram shown in Figure 4.
If a larger offset is desired, the offset voltage can be calculated as follows:
To calculate the correct biasing voltage, the maximum reflection should be
From laboratory measurements maximum V ref = 0.3Vp_ p . We are only
concerned with the negative portion of V rof = 0.15V
l ro< <= 0.15V/(R t1 |j R t2 )
l ref < = 2.5ma
where R,, = R t2 = 1 20 ohms = termination resistance
and R t = R t1 | j R t2
We want (1^,, - IfJR, > = 50mV minimum
The 50mV level is the hysteresis value for many RS-485 transceivers
therefore 1^., > = 3.33ma
To get the proper bias resistor values the following relationship can be used:
+ 5V = lbi«.(Rpull- up + ^pulldown + (Rtl II ^t2^
This yields a bias resistor value of 720 ohms for the entire network,
which gives a bias voltage of 0.35V.
EFFECTS OF BIAS ON REFLECTIONS
W/ NO BIAS
REFLECTION OCCURS (- 3V )
CAUSING FALSE TRANSITION
W/ 720 OHM BIAS RESISTORS
RAISES REFLECTION TO .1V
In practice, it is better to provide a larger bias resistor at each node so that the parallel
combination of all nodes can provide a proper offset voltage. A lower resistor value
than the suggested may be used if a large amount of noise is present in the system
or if larger reflections are encountered. It should be noted, that the biasing
arrangement adds to the total D.C. loading of the network, and yields a total load of
Rtoul = Rt1 ! i Rt2 I ! (Rrcvr/N) 1 1 (R bia8 /N)
where R t1 = R t2 = termination resistance
R rcvr = receiver impedance
Rbi«i = bias resistance
N = Number of Nodes
CALCULATING THE NUMBER OF NODES AND LENGTH OF CABLE
Three parameters must be taken into account when deciding upon the configuration
of the network (i.e. line length and number of nodes). These parameters are:
1) D.C. load
2) Cable attenuation
3) Noise margin
The first two parameters have been previously discussed and the third parameter,
noise margin, will be addressed now. The noise margin is the minimum voltage above
the 200mV receiver sensitivity limit prescribed by EIA RS-485. All further calculations
will assume a OV noise margin. Your individual application may require a greater noise
margin then that prescribed by the EIA RS-485 specification. The following
relationship may provide
= .8(V driver - V ta - V nob . - VaJ
where V.^ = voltage at end of line
v driver = driver output voltage
Vict = voltage loss due to cable
V no«. - noise margin
V bi« = D.C. bias voltage applied to network at each node
.8 = Derating for cable tolerances (± 20%)
The driver output voltage is a function of the D.C. load presented to the driver by the
network and can be calculated as described above. For convenience, Table 4 contains
the number of nodes vs. D.C. load using the typical transceiver characteristics for the
751 76B RS-485 transceiver.
V lo „ can be found by determining the driver output voltage for a given load and using
Table 2 to calculate the line loss for a given distance and frequency.
Usually, either the number of nodes required or the maximum cable distance are
known prior to designing a network. By using the above relationship all the remaining
variables (i.e. number of nodes or maximum cable length) can be determined and
combined to implement a working network.
BIASING THE NETWORK
From experimental results, a good value for biasing resistors is 810 ohms. This value
can be implemented in two ways. One way is to provide one set of resistors for the
entire network with a separate power supply for the bias. The second option is to
provide separate bias resistors at each node, thus providing greater flexibility in adding
nodes and increased reliability. This has the effect of increasing bias as nodes are
added, automatically compensating for the additional reflections generated, but
decreasing the dynamic range due to loading. SMC recommends the use of this
second option. Table 3 shows the recommended values for different numbers of
nodes. Note that no one value is optimal because as the node numbers change the
loading of the network will increase due to biasing.
Table 3 - Bias Resistance vs. Number of Nodes for 2.5 Mbs
Number of Nodes
1 - 10
21 - 30
31 - 40
Table 4 contains the estimated load and corresponding driver output voltage based on
the typical characteristics of the 75176B transceiver and the bias resistance from
Table 5. Note that these are typical characteristics and your driver may vary
significantly from these numbers.
Table 4 - D.C. Load Table
Estimated V OUT
Table 5 takes into account line loss and shows Table 4 with the maximum distance
that the network can be driven using the various data rates supported by the
COM20020 (i.e. 312.5Kbs, 625Kbs, 1.25Mbs, 2.5Mbs). A 0.1 volt noise margin
was used and a 0.200V receiving end voltage were used in establishing the criteria
for cable length. The maximum distance driven is relatively independent of load for
the node numbers of interest (<100). This is evidenced by examining Table 4 and
noticing that the output voltage does not vary significantly as the number of nodes
increases. Therefore, Table 5 shows the maximum distance using a driver output of
Table 5 - Line Len
gth vs. Maximum Number of Nodes
It should be noted that these figures are for the typical characteristics and represents
a nominal case. The worst case output voltage for a RS-485 transceiver is 1.5V
minimum. Typically, this worst case driver output is only encountered under heavy
load conditions. If your system exhibits this parameter, measurements should be
taken to correct for the lower driver output. A lower V driver will result in shorter line
lengths, however, if longer line length is needed less nodes can be accommodated.
Line capacitance can have a significant effect on the performance of the network.
The loading effect of the capacitance will affect both line length and the number of
Line capacitance is a combination of two sources. The first source is the effective
capacitance of the two twisted wires. This is the dominant source and is directly
proportional to line length (i.e as length increases so does C, ina ). The second source
is attributed to the capacitor formed from one wire to the insulation. This secondary
capacitance is often substantially smaller but cannot be ignored. In twisted shielded
cables, there is additional capacitance from conductor to the shield which can be quite
appreciable in some cable types.
Network performance is often degraded by line capacitance and depends on the
numbers of logical one's and zero's in a particular byte. Additionally, significant duty
cycle distortion can occur due to how much the line capacitance can charge before
it is discharged. For example, if a data byte of value 01 h is transmitted across a cable
in an RS-485 network, a significant amount of time is spent with the line voltage at
+ 5V differentially. This amount of time is enough for the line to fully charge. When
the logical one is driven (OV), the line must be completely discharged in order for the
voltage to fully reach is lowest level. Quite often in high data rate networks, the bit
time is much less than the time constant of the line. This will cause the duty cycle
of the bits to vary depending on the bit rate, line length, and data pattern. The more
logical one's in a given byte the better the duty cycle, because the line never gets to
fully charge due to the many one's that are being transmitted. A secondary effect of
line capacitance is bit jitter and is related to duty cycle distortion. Bit jitter results in
varying pulse widths being received at the RX input of the COM20020 device. If the
pulse width falls below 10ns then the bit is not detected by the COM20020 resulting
in framing and CRC errors.
If capacitive effects are significantly degrading a network two approaches can be
taken to resolve the problem. Solution one is to decrease the data rate of the network
so as to reduce the duty cycle distortion and bit jitter, and the second is to use a
special low capacitance cable specially designed for data communications. This type
of cable often has increased costs as compared to telephone grade cable and may not
be suitable for your every application.
CABLING TOPOLOGY AND CONNECTORS
SMC recommends a daisy-chained wiring scheme to reduce the reflections caused by
long stubs used in a trunk-drop topology. RJ-1 1 connectors should be used through
the network with a 1:1 wiring scheme (no reverse wiring). TEE connectors such as
MOD-TAP p/n S09-300-444 can be used for wiring convenience but the stub should
be kept as short as possible. If connectors other than RJ-1 1 are used measurements
should be taken to insure that there is no insertion loss due to the connector. If
insertion loss exists, the following equation can be used to calculate line length:
V = N/V - V, - V - V • - Vu: - (V- * N)
rcvr driver " I cms noise * margin * bias ' insertion 1 ' '
where V rcvr = voltage received at the end of the cable
Vic. = voltage loss from cable attenuation
Vow = driver output under D.C. load of network
V noi.e = noise margin
Vmrom = tolerance of cable ( + /- 20%)
Vbia. = Bias voltage applied to the network (typically .4v)
Vinsertion = insertion loss
N = number of nodes
There are several manufacturers of unshielded twisted pair cable. Here are a few
vendors of this type of cable:
For lengths greater than those listed in the above tables, SMC recommends inserting
RS-485 repeaters in-line or using a hub in a Star topology. Note that, when using a
hub or repeater, the device will act as one node. Be sure to take this into account
when designing the network.
NODE N + 1
EXPERIMENTAL VERIFICATION OF RS-485 CABLING GUIDELINES FOR
To establish a correlation between Distance and Number of Nodes (based on the
theory presented in the Cabling Guidelines) by observing network operation when
distance is varied and when the number of nodes is varied. The data rate is held
constant at 2.5 Mbps. Robust network operation is based on the following criteria:
a. Reflections should never cause the idle level to drop below -0.050 V,
b. All negative active levels should be more negative than -0.200 V,
c. Pulse widths of RXIN signals on various nodes should be greater than
d. A thorough transmit/receive test (denoted here as Ping Pong) should
operate successfully between the farthest nodes on the network and all
nodes in between.
The media used is 24 AWG solid copper unshielded 2-pair twisted pair with
characteristic impedance of 120 Q. The network is configured in a stand-alone bus
topology containing no repeaters. The cable is terminated using 120 Q resistors on
either end of the bus. The drops used to attach the nodes to the bus are MOD-TAP
part# S09-300-444, which have a drop length of 3 in. and negligible insertion loss.
The RS-485 transceivers used are National's 751 76B. If a different physical media
is to be evaluated, the criteria and methodology of this document may be followed.
If the usage of repeaters are to be evaluated, bit jitter could be aggravated, and it is
recommended to tighten the requirements on RXIN pulse width to approximately 1 80
nsec in order to protect against bit jitter error.
1. Configure the network to contain the maximum distance of cable specified in
the Cabling Guidelines (900 feet). Connect the maximum number of nodes to
the network which allows the criteria described above to be met. Attaching the
appropriate bias resistors to the nodes fights reflections but hinders the
minimum negative active level. Begin with 2.7 KQ bias resistors on each node.
To determine whether the measurements fall within the criteria, take
measurements as follows:
a. To differentially measure reflections, connect Channels 1 and 2 of the
oscilloscope to the cable interface pins of the RS485 transceiver (pins
6 and 7 on the 75LS176). Observe the signals differentially by setting
the oscilloscope to ADD and INVERT Channel 2. Connect the ground
lines of the scope probes together; do not connect to system ground.
Observe the tokens sent by each node to ensure that the idle level never
drops below -0.050V, measured differentially. Pay special attention to
the area just following the second DID of the token.
b. To differentially measure the minimum negative active levels, maintain
the same setup as in (a). Observe the tokens sent by each node to
ensure that negative active levels are always more negative than -
0.200V, measured differentially. Pay special attention to the token sent
by the furthest physical node from the one being observed. This token
should appear to have the most attenuation.
c. To measure the RXIN pulse width, place Channel 1 on the RXIN signal
of the COM20020 (pin 17 of the DIP package, pin 20 of the PLCC).
View Channel 1; this is not a differential measurement. It may help to
see each desired transmitted token by triggering externally off of the
TXEN of the transmitting node. Ensure that all RXIN pulses are present
and that they are all greater than 100 nsec wide. Due to the biasing of
the network, the capacitance of the cable, and the RS-485 transceivers,
the pulses most likely to disappear are the negative transitions (logic
"1 "s to the COM20020) immediately following a long duration of a high
level (many logic "0 n s to the COM20020). Pay special attention, for
instance, to the negative transition in the 04H pattern (End Of
Transmission pattern) and the negative transition immediately following
this pattern. Again, the token sent by the furthest node is most likely to
have problems as viewed by the observed node. Although the
COM20020 data sheet specifies a minimum RXIN width of 10 nsec, it
is important that the RXIN signals be greater than 100 nsec. This is
because bit jitter, which is affected by duty cycle distortion due to the
capacitance of the network, must be taken into account. As long as the
RXIN width is greater than 100 nsec, it is guaranteed that bit jitter will
not cause the receive circuitry of the COM20020 to lose
2. Record the results obtained in Step 1, including the worst reflections on the
network, minimum negative active level of the worst transition from the
furthest node, RXIN width of the worst transition from the furthest node, and
whether the Ping Pong test works between all nodes on network.
3. Detach 100 feet of cable, keep two nodes attached to network, and make the
same measurements as in Step 1 and record.
4. Add nodes to the network, remeasuring the parameters, until the
measurements are similar to those obtained previously (worst reflection
amplitude, minimum negative active level, RXIN width, and behavior of Ping
5. Repeat steps (3) and (4) until Distance vs. Number of Nodes curve is complete.
See Figure 1 for the setup of the experiment. The media used is 24 AWG solid
copper unshielded 2-pair twisted pair with characteristic impedance of 120 CI. The
network is configured in a stand-alone bus topology. The cable is terminated using
120 Q resistors on either end of the bus. The drops used to attach the nodes to the
bus are MOD-TAP part# S09-300-444, which have a drop length of 3 in. and
negligible insertion loss. The RS-485 transceivers used are National's 751 76B.
Scope views Minimum Negative Active Level of
Token passed by NIO FD by adding and Inverting
Channel 2. FDQN width Is viewed on Channel 3.
Table 1 - Results
(Each node containing 2.7 KC1 bias resistors)
No. of Additional Nodes to
Bring to Minimum Negative
Active Levels Previously
olMI/7UU/oUO/5HMl r eet
/ / /
1 . The Cabling Guidelines specify that 900 feet is the maximum distance for a
network. Experimentally, it was found that only 2 nodes may exist at 900 feet
to safely meet the criteria listed above.
2. 100 feet of cable was detached. At 800 feet, the addition of 4 more nodes
was required to bring the Minimum Negative Active Level to those obtained for
2 Nodes at 900 feet.
3. Another 100 feet of cable was detached. At 700 feet, the addition of 4 more
nodes was required to bring the Minimum Negative Active Level to those
obtained for 2 Nodes at 800 feet, and the addition of 10 for those obtained at
4. Another 100 feet of cable was detached. At 600 feet, the addition of 5 more
nodes was required to bring the Minimum Negative Active Level to those
obtained for 2 Nodes at 700 feet, the addition of 10 for those obtained at 800
feet, and the addition of 14 for those obtained at 900 feet.
5. A final 100 feet of cable was detached. At 500 feet, the addition of 5 more
nodes was required to bring the Minimum Negative Active Level to those
obtained for 2 Nodes at 600 feet, the addition of 10 for those obtained at 700
feet, and the addition of 14 for those obtained at 800. * When 18 additional
nodes were added, the minimum negative active level was similar to that
obtained for 800 feet, which means that as cable distance decreases, the
number of nodes begins to increase more rapidly.
;of 2.7 kOhm
100 200 300 400 500 600 700 800 900
Note that the previous values were all obtained using bias resistors of 2.7 KQ. Once
the node count exceeds 10, it is suggested to place higher value bias resistors on
each node. This brings the bias level down to a reasonable level, allowing the
minimum negative active level to become higher, but keeping the level of reflections
still within specification.
The Distance vs. Number of Nodes curve, obtained from the previous results (using
2.7 KQ bias resistors), is presented in Figure 2. Also in Figure 2 is a portion of the
curve which results in placing 12 KQ bias resistors on each node.
The following are additional results obtained, which may be helpful to the user,
including the Edge Rate (around OV differential) of the worst pulse seen from 20
unique tokens and the rise and fall time of a typical pulse. They are presented here:
Rising Transition Edge Rate = 22.2 Volts/^sec worst case.
Falling Transition Edge Rate = 7.27 Volts/^sec worst case.
Rise Time = 280 nsec typical.
Fall Time = 130 nsec typical.
It was seen that the curve was relatively linear when one bias resistor value is placed
on each node. Since the slope of the curve is approximately 4 nodes for every 100
feet (in terms of Minimum Negative Active Level), we can see that the addition of a
1 00 foot segment of cable has four times the impact versus the addition of one node.
Therefore, the addition of two more nodes at 600 feet does not significantly hinder
the network performance. In fact, connecting a total of 18 nodes at 600 feet with
2.7 KO bias resistors still showed perfect Ping Pong operation between all nodes. In
addition, 10 Nodes at 1000 feet was observed, with the following results:
Reflections of -0.100 V, a minimum negative active level of -0.200 V, and an RXIN
width of 40 nsec. Ping Pong operation was adequate, but measurements such as
these should be considered marginal because pushing the limits of the curve can
represent marginal behavior. At any time, a tradeoff exists between reflections and
the minimum negative active level. Adding many nodes to the network increases the
probability of significant reflections. To overcome this, one might be inclined to raise
the bias of the network. At some point, however, the minimum negative active level
will no longer fall within the specified limits of the RS-485 Receiver.
Alternatively, increasing the number of nodes increases the total bias of the network.
Therefore, to keep the minimum negative active level within specification, the bias
resistor of each node may be increased thus decreasing the overall bias of the
network. Increasing the bias resistors on each node to 1 2 KQ allowed a large number
of nodes to operate over a longer distance. However, please note that reflections are
more likely to cross the specified threshold. The results of this experiment indicate
that 14 nodes at 900 feet, or 19 nodes at 800 feet exhibited successful results. No
additional testing was performed above 19 nodes.
5 Mbs DC COUPLED RS-485 ANALYSIS
As a result of future customer requirements, an analytical and experimental analysis
was performed with ARCNET running at a 5Mbs data rate using an RS-485
compatible physical layer. This analysis will present line loss, reflection values,
biasing concerns, and a distance vs. load chart.
Using a symmetrical 5Mhz sine wave burst, the average line loss for UTP with 120
ohm terminations was found to be -2.0dB per 100 feet.
REFLECTIONS AND RAF
From experimental results, the maximum reflection measured was.50Vp. p . The
average RAF was approximately -15.0dB of the incident wave.
As discussed in TN 7-5, DC biasing of the network is necessary to avoid unwanted
transitions as a result of reflections generated along the line. Although the maximum
reflection is .5 volts, we are only concerned with the negative portion of the wave,
so for calculation of the bias resistors a value of .25V will be used. Due to the larger
reflections generated at 5Mbs, the following chart should be used:
NUMBER OF NODES
2 - 5
21 - 32
Note: One set of bias resistors can be used for the entire network. In this case, a
value of 470 ohms should be used.
Analytical performance is based on several measured parameters including line loss
and maximum reflections given a 5Vp. p input. The bias resistors calculated above
were determined based on the maximum reflection. If a smaller incident is to be used,
the maximum reflection can be calculated using the RAF. Three cases are considered
in the analysis: a nominal case in which the 485 transceiver outputs a + 5V signal;
a typical case where the driver outputs 2.4V; and a worst case where the driver
1 - 5
6 - 10
1 .1 Kft
21 - 32
Note: The above numbers are calculated and do not take into account bit jitter or
duty cycle distortion effects.
The procedure and criteria used for determining acceptable performance are identical
to those outlined in Addendum I (Experimental Verification of RS-485 Cabling
Guidelines for the COM20020). The only parameter that was changed was the bit
jitter budget, which was decreased from 100ns to 50ns due to the change in clock
speed. The following results were obtained:
Maximum distance - 600ft Maximum number of nodes - 17
The maximum distance achieved was found with a +5v driver output. The primary
reason for the decreased performance is attributed to duty cycle distortion as a result
of the biasing network.